There
is a nasty little bug with KompoZer in some releases which causes it to
crash with a segfault when accessing any submenu which has another menu
below that.
For some reason, despite a suitable patched version being available, the faulty version is included in the new 9.04 release.

For anyone else struggling with this problem, there is a fix. To read
the background on the issue, it is covered in the Ubuntu Launchpad bug
report at https://bugs.launchpad.net/ubuntu/+source/kompozer/+bug/263441

As this bug report can be a bit hard to follow and extract the info
needed, I have copied below the instructions which I passed to another
Ubuntu user in the Launchpad answers. I also used the same fix and am
writing this on the fixed KompoZer!

Go to System > Administration > Software Sources.

Click Software Sources and enter your password when requested.

Click the second tab (entitled Third Party Software). Click the "+
Add" button and in the box which opens enter the following line (this
is for 9.04, if using an earlier version of Ubuntu, simply change the
word jaunty for intrepid or hardy, depending on your version)-

Press enter.You will be asked for your password, so type that in
(note that nothing will show as you type in the password - but it is
working!).
Very quickly, the keyserver will be added and you can now update your software list with the following command in the terminal -

sudo apt-get update

Again, press enter, type in password when asked.
You will see the sources being updated - this may take a minute or two, depending on how recently you checked for updates.

OK, the basics are now all completed and you can close the terminal once all is done.

Now, head to your Synaptic Package Manager (System >
Administration > Synaptic Package Manager) and uninstall kompozer.
If you do not uninstall it, you will get an error at the next step.

Now Kompozer is removed, you can now install Kompozer again - but
this time you will find you are installing a different version - this
is the latest one which works correctly!